As an ISTP, your primary mode of living is focused internally, where you deal with things rationally and logically. Your secondary mode is external, where you take things in via your five senses in a literal, concrete fashion.
ISTPs have a compelling drive to understand the way things work. They're good at logical analysis, and like to use it on practical concerns. They typically have strong powers of reasoning, although they're not interested in theories or concepts unless they can see a practical application. They like to take things apart and see the way they work.
ISTPs have an adventuresome spirit. They are attracted to motorcycles, airplanes, sky diving, surfing, etc. They thrive on action, and are usually fearless. ISTPs are fiercely independent, needing to have the space to make their own decisions about their next step. They do not believe in or follow rules and regulations, as this would prohibit their ability to "do their own thing". Their sense of adventure and desire for constant action makes ISTPs prone to becoming bored rather quickly.
ISTPs are loyal to their causes and beliefs, and are firm believers that people should be treated with equity and fairness.
Although they do not respect the rules of the "System", they follow their own rules and guidelines for behavior faithfully. They will not take part in something which violates their personal laws. ISTPs are extremely loyal and faithful to their "brothers".
ISTPs like and need to spend time alone, because this is when they can sort things out in their minds most clearly. They absorb large quantities of impersonal facts from the external world, and sort through those facts, making judgments, when they are alone.
ISTPs are action-oriented people. They like to be up and about, doing things. They are not people to sit behind a desk all day and do long-range planning. Adaptable and spontaneous, they respond to what is immediately before them. They usually have strong technical skills, and can be effective technical leaders. They focus on details and practical things. They have an excellent sense of expediency and grasp of the details which enables them to make quick, effective decisions.
ISTPs avoid making judgments based on personal values - they feel that judgments and decisions should be made impartially, based on the fact. They are not naturally tuned in to how they are affecting others. They do not pay attention to their own feelings, and even distrust them and try to ignore them, because they have difficulty distinguishing between emotional reactions and value judgments. This may be a problem area for many ISTPs.
An ISTP who is over-stressed may exhibit rash emotional outbursts of anger, or on the other extreme may be overwhelmed by emotions and feelings which they feel compelled to share with people (often inappropriately). An ISTP who is down on themself will foray into the world of value judgments - a place which is not natural for the ISTP - and judge themself by their inability to perform some task. They will then approach the task in a grim emotional state, expecting the worst.
ISTPs are excellent in a crisis situations. They're usually good athletes, and have very good hand-eye coordination. They are good at following through with a project, and tying up loose ends. They usually don't have much trouble with school, because they are introverts who can think logically. They are usually patient individuals, although they may be prone to occasional emotional outbursts due to their inattention to their own feelings.
ISTPs have a lot of natural ability which makes them good at many different kinds of things. However, they are happiest when they are centered in action-oriented tasks which require detailed logical analysis and technical skill. They take pride in their ability to take the next correct step.
ISTPs are optimistic, full of good cheer, loyal to their equals, uncomplicated in their desires, generous, trusting and receptive people who want no part in confining commitments.
Jungian functional preference ordering:
Dominant: Introverted Thinking
Auxiliary: Extraverted Sensing
Tertiary: Introverted Intuition
Inferior: Extraverted Feeling